Typical materials used in implants to be used in medical treatment include metal, ceramic and polymer. Among these, metallic implants have superior mechanical properties and processability. However, metallic implants are disadvantageous because of stress shielding, image degradation and implant migration. Also, ceramic implants have superior biocompatibility compared to the other implants. However, ceramic implants are easily broken by external impact, and are difficult to process. Also, polymeric implants have relatively weak strength compared to the other implant materials.
Recently, porous implants are being developed which may accelerate the formation of bone tissue upon insertion into the human body and may decrease Young's modulus to prevent stress shielding. However, such porous implants have low mechanical strength and are weak to external impact. Also, research and development is being carried out into biodegradable implants which need not be removed after being inserted into the human body to achieve their desired purpose. The study of medical applications using such a biodegradable material has already begun since the middle of the 1960s and is mainly focused on using polymers such as polylactic acids (PLA), polyglycolic acid (PGA) or a copolymer thereof including PLGA. However, biodegradable polymers have low mechanical strength, produce acids upon decomposition, and have the disadvantage that it is difficult to control their biodegradation rate, and thus they have limited applications. In particular, the biodegradable polymers are difficult to apply to orthopedic implants that have to withstand a strong load or dental implants because of the properties of polymers having low mechanical strength. Hence, some biodegradable materials are being studied to overcome the problems of the biodegradable polymers. Typical examples thereof include ceramic such as tri-calcium phosphate (TCP), combination materials of biodegradable polymer and biodegradable hydroxyapatite (HA), etc.
However, mechanical properties of such materials are not much higher than those of biodegradable polymers. In particular, poor impact resistance of the ceramic material is regarded as very disadvantageous in a biomaterial. Also, the actual usability of such materials is open to question, because it is difficult to control the biodegradation rate.
Meanwhile, biodegradable implants should be very strong because part or all of it have to withstand a load when used into the human body. In order to ensure high strength, a biodegradable implant is further subjected to additional processes including rapid cooling, extrusion, and heat treatment so that the framework of the implant is made fine and internal residual stress should be controlled. Also, the alloy composition of a metal used for a biodegradable implant should be appropriately designed by changing constituent elements or content thereof. As such, changing the alloy composition may be typically performed by adjusting the amounts of the elements that are added. As the amounts of elements added to the alloy increase, mechanical strength is enhanced.
However, when the amounts of added elements are increased, the metal for the implants may easily create a galvanic circuit that increases the corrosion rate attributable to an increase in the non-uniformity of the composition thereof and the non-uniformity of a fine framework, undesirably increasing the corrosion rate of implants. Hence, it is very difficult to design alloy materials which have high strength and low biodegradation rate to be applied to implants.